All the corporeal terms used in the Scriptures to describe Him -- such as walking, standing, sitting, speaking etc. -- are metaphorical. As the Sages have said: "The Torah speaks in the language of man."
This is the third Principle, as affirmed by the verse (Deuteronomy 4:15) "You have not seen any image," that is to say, you cannot conceive of Him as having any form because, as stated, He is neither a body nor a bodily force. (italics mine) This ikkar is going to be one of the easiest of all. I certainly believe it. It states generally that God is incorporeal, even though Scripture generally portrays Him in most corporeal terms. That, says the Rambam, is simply anthropomorphism, which is a perfectly acceptable literary device (I say that).
What is difficult about the Rambam's assertion is that his few prooftexts attest only that God doesn't have a body or that specific people didn't see God. One might think that the abundance of anthropomorphic depictions of Hashem in the Torah suggest otherwise. And "one might think" is the same as "many people think". R. Avraham ibn Daud took the Rambam to task for suggesting that it is heresy to believe God is corporeal, saying that many people who were "even greater" than the Rambam believed God was corporeal.
Be that as it may, it is well attested in ancient sources that the Jews did not believe that God has a body. This was a specific feature of Judaism that apparently frustrated pagans. In a time before microscopes revealed the hidden world of perfectly corporeal and tangible things that are all around us which we cannot see, it was difficult to conceive of a god that is neither physical nor even "spiritual" ala demons and angels, and yet exists.
All kinds of inventive theories were advanced about the true nature of the Jewish religion and the Jewish deity, including the strange belief about Jewish "ass worship" (hint: don't look that up in google).
So why would I believe it, other than the fact the Rambam says to believe it? Or would I believe it if he hadn't? After all, the Torah strongly suggest otherwise, despite single verses to the contrary. I don't honestly know, really. I do know that whether its because I've been socialized to this belief or not, a corporeal God strikes me as immature. It isn't as if the Torah presents a uniform anthropomorphic image of God. God appears in the guise of a mighty warrior, (possibly) as angels, as pillars of fire, as having arms, as a still, small voice and more. To me, all things is the same as saying no things. Therefore it seems like the Rambam is correctly reading these as metaphors, prodded on and supported by the pesukkim that suggest that Hashem really has no body and really cannot be seen.